id Tech 4 began as an enhancement to id Tech 3. During development, it was initially just a complete rewrite of the engine's renderer, while still retaining other subsystems, such as file access, and memory management. The decision to switch from C to the C++programming language necessitated a restructuring and rewrite of the rest of the engine; today, while id Tech 4 contains code from id Tech 3, much of it has been rewritten.[3]

At the QuakeCon 2007, John Carmack, the lead graphics engine developer at id, said to LinuxGames: "I mean, I won't commit to a date, but the Doom 3 stuff will be open source". And like its predecessors, John Carmack has said that id Tech 4 will be released as open source.[4]

At the QuakeCon 2009, Carmack said that he planned to petition ZeniMax Media to release the id Tech 4 source upon the release of Rage.[5] As part of his keynote for QuakeCon 2011 he confirmed that the source code to Doom 3 will be out by the end of the year, after the release of Rage.[6]

On November 16, 2011, Carmack announced on Twitter that he's writing new code for Doom 3's open source release, because "lawyers are still skittish about the patent issue around 'Carmack's reverse'".[7] This refers to an implementation of stencil bufferedshadow volume algorithms.

On November 22, 2011, Carmack released the Doom 3 source code on GitHub.[8] The source does not include any of the 'Carmack's Reverse' code. According to Carmack, minor tweaks were made to the code to avoid any infringement.[9]

The original requirement of id Tech 4 was that it needed a high-end graphics processing unit (GPU) with fully programmable vertex and pixel shaders, such as the NvidiaGeForce 3 or ATIRadeon 8500, with at least 64 MB of VRAM. By E3 2002, the recommended GPU was "100% DirectX 9.0b compatible", such as the Radeon 9700 with 128 MB of VRAM. While the Radeon 9700's DirectX 9.0 features are not necessary to render the game, its advanced architecture, 256-bit memory bus, and efficiency were needed to run Doom 3 at high detail and playable speed.[10] The "Ultra" graphics mode included in Doom 3 would not even run on the current popular graphics cards available in 2004, requiring at least 512 MB of video memory to display properly and at playable speeds, meaning that it was basically unusable at the time of release.[11]

id Tech 4 resulted in the obsolescence of DirectX 7 graphics chips such as the widespread GeForce 2 and Radeon 7200, as well as older chipsets such as RIVA TNT2 and Rage 128, and software rendering (with an integrated Intel GMA). Until the advent of id Tech 4, a powerful CPU was able to somewhat compensate for an older video card. While John Carmack initially warned gamers not to purchase the GeForce 4 MX[12] (which casual consumers often confused with the DirectX 8 capable GeForce 4 Ti, though it was at best an improved GeForce 2), its somewhat widespread adoption compelled id Software to add it to the list of supported cards. There have been cases of enthusiasts forcing Doom 3 to run on unsupported graphics chips, such as the long obsolete Voodoo 2, but these are unable to render the per-pixel lighting and bump mapping.[13]

Id Tech 4 added several new graphical features absent in its predecessor, id Tech 3. These included bump mapping, normal mapping, and specular highlighting. More features were added in the development of successive games, and in yet unreleased games using id Tech 4, new features have been added or are planned to be added soon.

The primary innovation of id Tech 4 was its use of entirely dynamic per-pixel lighting, whereas previously, 3D engines had relied primarily on pre-calculated per-vertex lighting or lightmaps and Gouraud shading. While dynamic effects had been available before (such as dynamic moving lights), this effect merely changed the brightness of the vertices of the polygon, with the pixel's colors simply being interpolated between the three vertex colors of its polygon.

This fully realtime approach used in Doom 3, combined with the use of shadow volumes permitted more realistic lighting and shadows[14] than in the previous generation of id's engines. The method used to create the shadow volumes is the subject of a patent by Creative, which Creative granted id permission to use in the Doom 3 engine, in exchange for supporting Creative's EAX advanced sound technologies.[15]

The models used in id Tech 4 engine games are animated using skeletal animation. The engine can blend multiple animations together, to produce a skin that moves correctly for those animations. Because this is CPU intensive, id did some work optimising this by using Intel's Streaming SIMD Extensions (SSE).[16]

The original version of the id Tech 4 engine was designed for somewhat dark environments so it was criticized for its perceived inability to handle extremely large day time outdoor areas. The MegaTexture technology not only removed this issue by introducing a means to create expansive outdoor scenes but also made the new version as the best game engine to handle the outdoor areas, as well[citation needed]. By painting a single massive texture (32,768×32,768 pixels, though it has been extended to larger dimensions in recent versions of the MegaTexture technology) covering the entire polygon map and highly detailed terrain, the desired effects can be achieved. The MegaTexture can also store physical information about the terrain such as the amount of traction in certain areas or indicate what sound effect should be played when walking over specific parts of the map. i.e. walking on rock will sound different from walking on grass.[17] It is expected that this will result in a considerably more detailed scene than the majority of existing technologies, using tiled textures, allow. The first game utilizing MegaTexture is Enemy Territory: Quake Wars, based on the Tech 4 engine. All id Tech 5 games use MegaTexture.

id Tech 4 has a comprehensive scripting language which can be used when creating mods, and is used in Doom 3 to control monsters, weapons, and map events. This scripting language is similar to C++.[18]

In addition to the main scripting language, idTech 4 also has another scripting language that is used for GUIs - both the menus and HUD, and also for GUIs embedded into the game world.[19] These in-game GUIs are sufficiently powerful that you can, for example, run another game such as Doom 1 within the game-world.[20]

Despite this additional level of scripting, it is also possible to create mods using C++ to build native code.[21]

As a result of the agreement with Creative regarding the patent on shadow volumes, the id Tech 4 engine supports OpenAL, including the Creative Technology specific EAX extensions. The work to include OpenAL support was done by Creative Technology, not by id themselves.[22]

The engine uses a traditional client–server model. Initially it was planned to have a peer-to-peer networking model.[23] This part of the engine works in a fundamentally similar way to the id Tech 3 equivalent, however id Tech 4 exposes a lot more of the network protocol to mod developers.[24]

iodoom3 is a game engine project which intends to take advantage of building on id Software's id tech 4 source code release. A followup to ioquake3, the project was started shortly after the source code release with the goal of creating a bug-free, enhanced open-source id tech 4 source code distribution upon which new games and projects can be based.[25]

dhewm3 is another game engine project of the id Tech 4 GPL source code. The goal of this engine is bring id Tech 4 with the help of SDL to all suitable platforms, and fix the bugs existing without altering the original gameplay.[26]

The main changes of dhewm3 compared with the original id Tech 4 engine are 64bit port, SDL for low level OS support, OpenAL for audio output including OpenAL EFX for EAX reverb effects and a portable build system based on CMake.[26]

RBDOOM-3-BFG is a game engine project based upon the source code release of the Doom 3 BFG Edition in 2012. Additional features include Linux and Windows 32/64bit support, OS X support, Bink video support through FFmpeg, Soft shadows using PCF hardware shadow mapping and modified shaders for lighting interaction to use Half-Lambert lighting like in Half-Life 2. [27]

Prey utilized a modified version of id Tech 4 that allowed for effects such as portals, variable gravity, and wall walking

Compared to the preceding and widely used id Tech 3 (Quake III Arena engine) and id Tech 2 (Quake II engine), id Tech 4 has had less success in licensing to third parties.[28] This is especially apparent in comparison to its closest contemporaries, Unreal Engine 2 (2002) and Unreal Engine 3 (2006) from Epic Games.[29] id Software was unwilling to license their newest engine before its "parent game" Doom 3 was completed. However the unexpected long development time going into Doom 3 from 2002-04 meant that they could not put up competition to Epic Games's Unreal Engine 2 during that period. Many who licensed Unreal Engine 2 were thus able to make the switch to Unreal Engine 3 more easily.

While id Tech 4 had taken a new direction with its dynamic per-pixel lighting, this unconventional feature had steeper hardware requirements and was initially only useful in "spooky games" (until the MegaTexture addition), whereas an increasing number of developers preferred conventional engines that could render large outdoor areas. Also notable was id Tech 4's relative lack of downward scalability compared to competing FPS engines; id Tech 4 generally required a DirectX 8.0 compliant GPU such as a GeForce 3; the rival Source engine (which was developed from the previous GoldSrc engine) could still run on the older widespread DirectX 7 GPUs (albeit without shaders being used).